Suction-operating volumetric dosers are know, in which each dosing chamber is defined by a filtering member or screen through which said chamber is firstly, subjected to vacuum to cause the powder material to enter the chamber and fill it completely with a desired degree of compaction, and secondly, subjected to pressure to eject the dosed material. Heretofore, said screens have been made either of small discs of sintered stainless steel, 2-3 mm. thick and having interstices ranging in amplitude from 14 to 160 microns, or of small discs of felt approximately 4 mm. thick and having interstices ranging within a few microns. If these screens are used for dosing finely powdered materials, such as the powder pharmaceuticals, such as antibiotics, said screens will become clogged quickly, with obvious consequences.
The reasons that make said small discs of sintered stainless steel or felt unsuitable to constitute filtering screens for fine powder material, are of various kinds and can be summarized as follows:
randomly-scattered sizes and spacing of the interstices or pores through which the air stream passes;
tortuosity of said interstices, which results in considerable loss of head, so as to require relatively high levels of suction and compression, which on turn requires discs of relatively high thickness to withstand the mechanical stresses resulting from either the suction and compression reciprocating air streams to which they are subjected.
To solve this problem, therefore, a filtering screen should present perfectly calibrated uniform and evenly-distributed interstices, said interstices being, moreover, of limited length. Several efforts have been made for this purpose, by providing wire mesh screens made with thin wires of stainless steel and complying with said requirements, but said screens had a poor resistance to the mechanical stresses to which they were submitted by the suction and compression air streams mentioned above. Efforts were made, therefore, to develop diaphragms of mesh formation having a higher resistance to mechanical stresses, and according to the invention it was found that the problem is solved satisfactorily by means of a fabric of synthetic material, of mono-filament structure, that is wherein the warp and the weft are constituted by mono-filaments and not by a spun yarn, and which is, at the same time, of sterilizable and non-toxic nature. Such a fabric can be made of a material such as the polytetrafluoroethylene resin commercially known under the registered trade mark "Teflon" or the like. The interstices of the fabric, obviously, shall have a width which is conveniently smaller than the fineness of the powder material to be handled.
Secondarily, the invention solves the problems of the correct positioning of the said fabric screen in its operative place, and of a quick and easy substitution of said fabric screen, whenever required.
The above and other advantages of the filtering member or screen according to the invention, will appear evident from the following description of a preferred embodiment of same.